Plants of the aforementioned kind for manufacturing cement clinker are generally known from the literature.
The emission of SO2 from such modern kiln plants for manufacturing cement clinker is normally relatively low due to the fact that the sulphur contained in the fuel input for the kiln and for any calciner is bound very effectively in the form of sulphate which is discharged from the kiln embedded in the clinker. However, a certain emission from the preheater may occur if the utilized raw materials contain sulphide as is the case in the frequently occuring minerals pyrite and marcasite.
The reason for this is that pyrite FeS2 is decomposed in the preheater at temperatures around 550° C. according to the equation:FeS2=FeS+S  (1)
whereafter the evaporated S is immediately converted by burning into SO2. FeS is somewhat more resistant and reaches the calcining zone before it is burned, and the SO2 thus generated is subsequently bound by CaO in similar way as that produced from the fuel. This happens in accordance with the reaction equation:SO2+CaO+½O2=CaSO4  (2)
In case of a pyrite content in the raw materials, there is a risk that about one half of the sulphur content may escape in the form of SO2.
To reduce the extent of any such SO2 escape, it is known practice to introduce an absorbent in the form of CaO, Ca(OH)2 or other basic components at some location in the preheater so that SO2 can be bound in the form of sulphite:CaO+SO2=CaSO3  (3)
At a subsequent stage of the process, sulphite will be converted into sulphate.
A significant disadvantage of this known method is that it involves use of a surplus amount of absorbent, making the method relatively costly, particularly if the absorbent which is being used has to be purchased from an external source.
From WO 93/10884 a method is known by which exhaust gases containing CaO-laden dust are extracted from a location near the calciner and directed to the location in the preheater where the absorption of SO2 is to take place. The method works, but it requires a quite substantial surplus amount of CaO, thus somewhat reducing the efficiency of the preheater as a heat exchange unit.
The Danish patent application No. PA 1999 00867 provides an improved method by which calcined cement raw meal is extracted, slaked and ground before it is introduced into the preheater. However, this method also has the disadvantage of reduced efficiency of the preheater as a heat exchange unit.